U.S. patent application number 10/969102 was filed with the patent office on 2006-04-20 for retransmission scheme in a cellular communication system.
This patent application is currently assigned to IPWireless, Inc.. Invention is credited to Nicholas W. Anderson, Martin W. Beale.
Application Number | 20060084389 10/969102 |
Document ID | / |
Family ID | 35429304 |
Filed Date | 2006-04-20 |
United States Patent
Application |
20060084389 |
Kind Code |
A1 |
Beale; Martin W. ; et
al. |
April 20, 2006 |
Retransmission scheme in a cellular communication system
Abstract
An apparatus 300 for a cellular communication system (100)
comprises a buffer (303) which receives data for transmission over
an air interface (115). The buffer (303) is coupled to a scheduler
(305) which schedules the data and allocates the physical resource
of the air interface (115). The transmissions are performed using a
retransmission scheme such as a Hybrid-Automatic Repeat reQuest
scheme. A load processor (309) determines a load characteristic
associated with the scheduler (305) and a target controller (311)
sets a target parameter for the retransmission scheme in response
to the load characteristic. Specifically, a block error rate target
may be set in response to a load level of a cell or plurality of
cells. A transmission controller (307) sets a transmission
parameter for a transmission in response to the target parameter
and a transmitter (301) transmits the data using the transmission
parameter. Accordingly, an operating point of the retransmission
scheme may be dynamically adjusted thereby reducing overall
latency.
Inventors: |
Beale; Martin W.; (Bristol,
GB) ; Anderson; Nicholas W.; (Bristol, GB) |
Correspondence
Address: |
MORRISON & FOERSTER LLP
425 MARKET STREET
SAN FRANCISCO
CA
94105-2482
US
|
Assignee: |
IPWireless, Inc.
San Bruno
CA
|
Family ID: |
35429304 |
Appl. No.: |
10/969102 |
Filed: |
October 19, 2004 |
Current U.S.
Class: |
455/67.11 |
Current CPC
Class: |
H04W 28/14 20130101;
H04L 47/14 20130101; H04L 1/0009 20130101; H04L 1/0003 20130101;
H04W 52/34 20130101; H04W 84/042 20130101; H04L 1/0021 20130101;
H04W 28/02 20130101; H04W 72/0486 20130101; H04W 24/00 20130101;
H04L 1/1812 20130101; H04L 1/1887 20130101 |
Class at
Publication: |
455/067.11 |
International
Class: |
H04B 17/00 20060101
H04B017/00 |
Claims
1. An apparatus for a cellular communication system, the apparatus
comprising: a scheduler for scheduling data for transmission over
an air interface of the cellular communication system using a
retransmission scheme; load means for determining a load
characteristic associated with the scheduler; means for setting a
target parameter for the retransmission scheme in response to the
load characteristic; and means for setting a transmission parameter
for a transmission in response to the target parameter.
2. The apparatus claimed in claim 1 wherein the transmission
parameter comprises a message transmit power.
3. The apparatus claimed in claim 1 wherein the transmission
parameter comprises a transmit power reference indication.
4. The apparatus claimed in claim 1 wherein the target parameter
comprises an error rate.
5. The apparatus claimed claim 4 wherein the error rate is a Block
Error Rate (BLER).
6. The apparatus claimed in claim 1 wherein the transmission
parameter comprises a modulation parameter.
7. The apparatus claimed in claim 1 wherein the transmission
parameter comprises an error coding parameter.
8. The apparatus claimed in claim 1 wherein the transmission
parameter comprises a transmission parameter set restriction.
9. The apparatus claimed in claim 1 wherein the transmission
parameter is a transmission parameter of an initial transmission of
a message.
10. The apparatus claimed in claim 1 wherein the probability of a
retransmission is dependent on the setting of the transmission
parameter.
11. The apparatus claimed in claim 1 wherein the load means is
operable to determine the load characteristic in response to an
amount of pending transmit data.
12. The apparatus claimed in claim 11 wherein the pending transmit
data is associated with a single cell.
13. The apparatus claimed in claim 12 wherein the pending transmit
data is associated with a radio controller common for a plurality
of cells.
14. The apparatus claimed in any previous claim 11 to 13 wherein
the amount of pending data corresponds to a transmit data buffer
loading.
15. The apparatus claimed in claim 1 wherein the load means is
operable to determine the load characteristic in response to a
number of attached remote units.
16. The apparatus claimed in claim 1 wherein the transmission
parameter is biased towards an increasing number of retransmissions
for an increasing load.
17. The apparatus claimed in claim 1 further comprising means for
measuring a performance characteristic; and wherein the means for
setting the target parameter is further operable to set the target
parameter in response to the performance characteristic.
18. The apparatus claimed in claim 1 wherein the transmission is a
downlink transmission.
19. The apparatus claimed in claim 1 wherein the transmission is an
uplink transmission.
20. The apparatus claimed in claim 19 wherein the load means is
operable to determine the load characteristic in response to a load
indication received from a remote unit.
21. The apparatus claimed in claim 19 further comprising means for
transmitting an indication of the target parameter from a base
station to a remote unit.
22. The apparatus claimed in claim 19 further comprising means for
transmitting an indication of the transmission parameter from a
base station to a remote station.
23. The apparatus claimed in claim 1 wherein the means for setting
the transmission parameter is comprised in a base station, and the
means for setting the target parameter is comprised in a radio
network controller which is operable to communicate an indication
of the target parameter to the base station.
24. The apparatus claimed in claim 1 further comprising means for
communicating the target parameter to a scheduling function
associated with a different cell than a cell associated with the
means for determining the target parameter.
25. The apparatus claimed in claim 1 further comprising means for
communicating the load characteristic to a scheduling function
associated with a different cell than the means for determining the
load characteristic parameter.
26. The apparatus claimed in claim 1 wherein the retransmission
scheme is a Hybrid-Automatic Repeat reQuest (H-ARQ) scheme.
27. The apparatus claimed in claim 1 wherein the cellular
communication system complies with the Technical Specifications of
the 3.sup.rd Generation Partnership Project (3GPP).
28. A base station for a cellular communication system; the base
station comprising: a scheduler for scheduling data for
transmission over an air interface of the cellular communication
system using a retransmission scheme; load means for determining a
load characteristic associated with the scheduler; means for
setting a target parameter for the retransmission scheme in
response to the load characteristic; and means for setting a
transmission parameter for a transmission in response to the target
parameter.
29. A method of operation in a cellular communication system
including at least a scheduler for scheduling data for transmission
over an air interface of the cellular communication system using a
retransmission scheme, the method comprising: determining a load
characteristic associated with the scheduler; setting a target
parameter for the retransmission scheme in response to the load
characteristic; and setting a transmission parameter for a
transmission in response to the target parameter.
30. The method claimed in claim 29 wherein the transmission
parameter comprises a message transmit power.
31. The method claimed in claim 29 wherein the transmission
parameter comprises a transmit power reference indication.
32. The method claimed in claim 29 wherein the target parameter
comprises an error rate.
33. The method claimed in claim 32 wherein the error rate is a
Block Error Rate (BLER).
34. The method claimed in claim 29 wherein the transmission
parameter comprises a modulation parameter.
35. The method claimed in claim 29 wherein the transmission
parameter comprises an error coding parameter.
36. The method claimed in claim 29 wherein the transmission
parameter comprises a transmission parameter set restriction.
37. The method claimed in claim 29 wherein the transmission
parameter is a transmission parameter of an initial transmission of
a message.
38. The method claimed in claim 37 wherein the probability of a
retransmission is dependent on the setting of the transmission
parameter.
39. The method claimed in claim 29 wherein the determining of the
load characteristic comprises determining the load characteristic
in response to an amount of pending transmit data.
40. The method claimed in claim 39 wherein the pending transmit
data is associated with a single cell.
41. The method claimed in claim 40 wherein the pending transmit
data is associated with a radio controller common for a plurality
of cells.
42. The method claimed in any previous claim 38 wherein the amount
of pending data corresponds to a transmit data buffer loading.
43. The method claimed in claim 29 wherein the determining of the
load characteristic comprises determining the load characteristic
in response to a number of attached remote units.
44. The method claimed in claim 29 wherein the transmission
parameter is biased towards an increasing number of retransmissions
for an increasing load.
45. The method claimed in claim 29 further comprising measuring a
performance characteristic; and wherein the setting of the target
parameter comprises setting the target parameter in response to the
performance characteristic.
46. The method claimed in claim 29 wherein the transmission is a
downlink transmission.
47. The method claimed in claim 29 wherein the transmission is an
uplink transmission.
48. The method claimed in claim 47 wherein the determining of the
load characteristic comprises determining the load characteristic
in response to a load indication received from a remote unit.
49. The method claimed in claim 47 further comprising transmitting
an indication of the target parameter from a base station to a
remote unit.
50. The method claimed in claim 47 further comprising transmitting
an indication of the transmission parameter from a base station to
a remote station.
51. The method claimed in claim 29 wherein the setting of the
transmission parameter is performed by a base station, and the
setting of the target parameter is performed by a radio network
controller which further communicates an indication of the target
parameter to the base station.
52. The method claimed in claim 29 further comprising communicating
the target parameter to a scheduling function associated with a
different cell than a cell associated with the determining of the
target parameter.
53. The method claimed in claim 29 further comprising communicating
the load characteristic to a scheduling function associated with a
different cell than the determining of the load characteristic
parameter.
54. The method claimed in claim 29 wherein the retransmission
scheme is a Hybrid-Automatic Repeat reQuest (H-ARQ) scheme.
55. The method claimed in claim 29 wherein the cellular
communication system complies with the Technical Specifications of
the 3.sup.rd Generation Partnership Project (3GPP).
56. A computer-readable medium storing executable instructions for
operation in a cellular communication system including at least a
scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme, comprising instructions for: determining a
load characteristic associated with the scheduler; setting a target
parameter for the retransmission scheme in response to the load
characteristic; and setting a transmission parameter for a
transmission in response to the target parameter
57. A record carrier comprising a computer program as claimed in
claim 56.
58. An apparatus for a cellular communication system, the apparatus
comprising: a scheduler for scheduling data for transmission over
an air interface of the cellular communication system using a
retransmission scheme; load means for receiving a load
characteristic associated with the scheduler; means for setting a
target parameter for the retransmission scheme in response to the
load characteristic; and means for setting a transmission parameter
for a transmission in response to the target parameter.
59. An apparatus for a cellular communication system, the apparatus
comprising: a scheduler for scheduling data for transmission over
an air interface of the cellular communication system using a
retransmission scheme; means for receiving a target parameter for
the retransmission scheme, the target parameter being dependent on
a load characteristic associated with the scheduler; and means for
setting a transmission parameter for a transmission in response to
the target parameter.
60. An apparatus for a cellular communication system including a
scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme, the apparatus comprising: load means for
receiving a load characteristic associated with the scheduler;
means for setting a target parameter for the retransmission scheme
in response to the load characteristic; and means for setting a
transmission parameter for a transmission in response to the target
parameter.
61. An apparatus for a cellular communication system including a
scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme, the apparatus comprising: means for
receiving a target parameter for the retransmission scheme, the
target parameter being dependent on a load characteristic
associated with the scheduler; and means for setting a transmission
parameter for a transmission in response to the target
parameter.
62. An apparatus for a cellular communication system including a
scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme, the apparatus comprising: load means for
receiving a load characteristic associated with the scheduler; and
means for setting a target parameter for the retransmission scheme
in response to the load characteristic, the target parameter being
indicative of a setting of a transmission parameter for a
transmission.
63. A method for a cellular communication system including a
scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme, the method comprising: receiving a load
characteristic associated with the scheduler; setting a target
parameter for the retransmission scheme in response to the load
characteristic; and setting a transmission parameter for a
transmission in response to the target parameter.
64. A method for a cellular communication system including a
scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme, the method comprising: receiving a target
parameter for the retransmission scheme, the target parameter being
dependent on a load characteristic associated with the scheduler;
and setting a transmission parameter for a transmission in response
to the target parameter.
65. A method for a cellular communication system including a
scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme, the method comprising: determining a load
characteristic associated with the scheduler; and setting a target
parameter for the retransmission scheme in response to the load
characteristic, the target parameter being indicative of a setting
of a transmission parameter for a transmission.
Description
FIELD OF THE INVENTION
[0001] The invention relates to an apparatus, a base station and a
method for controlling at least one transmission of a
retransmission scheme in a cellular communication system.
BACKGROUND OF THE INVENTION
[0002] In a cellular communication system, a geographical region is
divided into a number of cells each of which is served by base
stations. The base stations are interconnected by a fixed network
which can communicate data between the base stations. A mobile
station is served via a radio communication link from the base
station of the cell within which the mobile station is
situated.
[0003] A typical cellular communication system extends coverage
over an entire country and comprises hundreds or even thousands of
cells supporting thousands or even millions of mobile stations.
Communication from a mobile station to a base station is known as
the uplink, and communication from a base station to a mobile
station is known as the downlink.
[0004] The fixed network interconnecting the base stations is
operable to route data between any two base stations, thereby
enabling a mobile station in a cell to communicate with a mobile
station in any other cell. In addition, the fixed network comprises
gateway functions for interconnecting to external networks such as
the Internet or the Public Switched Telephone Network (PSTN),
thereby allowing mobile stations to communicate with landline
telephones and other communication terminals connected by a
landline. Furthermore, the fixed network comprises much of the
functionality required for managing a conventional cellular
communication network including functionality for routing data,
admission control, resource allocation, subscriber billing, mobile
station authentication etc.
[0005] Currently, the most ubiquitous cellular communication system
is the 2nd generation communication system known as the Global
System for Mobile communication (GSM). GSM uses a technology known
as Time Division Multiple Access (TDMA) wherein user separation is
achieved by dividing frequency carriers into 8 discrete time slots,
which individually can be allocated to a user. Further description
of the GSM TDMA communication system can be found in `The GSM
System for Mobile Communications` by Michel Mouly and Marie
Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN
2950719007.
[0006] Currently, 3rd generation systems are being rolled out to
further enhance the communication services provided to mobile
users. The most widely adopted 3rd generation communication systems
are based on Code Division Multiple Access (CDMA) technology. Both
Frequency Division Duplex (FDD) and Time Division Duplex (TDD)
techniques employ this CDMA technology. In CDMA systems, user
separation is obtained by allocating different spreading and
scrambling codes to different users on the same carrier frequency
and in the same time intervals. In TDD, additional user separation
is achieved by assigning different time slots to different users
similarly to TDMA. However, in contrast to TDMA, TDD provides for
the same carrier frequency to be used for both uplink and downlink
transmissions. An example of a communication system using this
principle is the Universal Mobile Telecommunication System (UMTS).
Further description of CDMA and specifically of the Wideband CDMA
(WCDMA) mode of UMTS can be found in `WCDMA for UMTS`, Harri Holma
(editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN
0471486876.
[0007] In a 3rd generation cellular communication system, the
communication network comprises a core network and a Radio Access
Network (RAN). The core network is operable to route data from one
part of the RAN to another, as well as interfacing with other
communication systems. In addition, it performs many of the
operation and management functions of a cellular communication
system, such as billing. The RAN is operable to support wireless
user equipment over a radio link of the air interface. The RAN
comprises the base stations, which in UMTS are known as Node Bs, as
well as Radio Network Controllers (RNC) which control the base
stations and the communication over the air interface.
[0008] The RNC performs many of the control functions related to
the air interface including radio resource management and routing
of data to and from appropriate base stations. It further provides
the interface between the RAN and the core network. An RNC and
associated base stations are known as a Radio Network Subsystem
(RNS).
[0009] 3rd generation cellular communication systems have been
specified to provide a large number of different services including
efficient packet data services. For example, downlink packet data
services are supported within the 3GPP release 5 specifications in
the form of the High Speed Downlink Packet Access (HSDPA) service.
A High Speed Uplink Packet Access (HSUPA) feature is also in the
process of being standardised. This uplink packet access feature
will adopt many of the features of HSDPA.
[0010] In accordance with the 3GPP specifications, the HSDPA
service may be used in both Frequency Division Duplex (FDD) mode
and Time Division Duplex (TDD) mode.
[0011] In HSDPA, transmission code resources are shared amongst
users according to their traffic needs. The base station or
"Node-B" is responsible for allocating and distributing the
resources to the users, within a so-called scheduling task. Hence,
for HSDPA, some scheduling is performed by the RNC whereas other
scheduling is performed by the base station. Specifically, the RNC
allocates a set of resources to each base station, which the base
station can use exclusively for high speed packet services. The RNC
furthermore controls the flow of data to and from the base
stations. However, the base station schedules transmissions to the
mobile stations that are attached to it, operates a retransmission
scheme, controls the coding and modulation for transmissions to and
from the mobile stations and transmits (for HSDPA) and receives
(for HSUPA) data packets from the mobile units.
[0012] HSDPA and HSUPA seek to provide packet access techniques
with a relatively low resource usage and with low latency.
[0013] Specifically, HSDPA and HSUPA use the following techniques
in order to reduce the resource required to communicate the data
thereby increasing the capacity of the communication system: [0014]
Adaptive Coding and Modulation. The coding and modulation schemes
may dynamically be selected to be optimised for the current radio
conditions thereby providing effective link adaptation. For
example, in HSDPA, the 16 QAM higher order modulation may be used
to increase throughput for users in favourable radio conditions
whereas the less efficient but more robust QPSK modulation may be
used at less favourable radio conditions. [0015] Retransmission
with soft combining. HSDPA and HSUPA use a retransmission scheme
known as a Hybrid-Automatic Retransmission reQuest (H-ARQ) scheme
wherein retransmissions are soft combined with previous
transmissions in order to achieve an efficient communication. The
H-ARQ scheme is typically operated at a higher block error rate for
individual transmissions in order to increase efficiency, but the
final block error rate after soft combining is similar to the block
error rate for pre-HSDPA systems. [0016] Fast scheduling is
performed at the base station. This allows scheduling to be
sufficiently fast to dynamically follow radio condition variations.
For example, when more than one mobile unit requires service, the
base station may schedule data to the mobile stations experiencing
favourable radio conditions in preference to the mobile stations
experiencing less favourable conditions. Furthermore, the resources
allocated and the coding and modulation applied to transmissions to
mobile stations may be highly tailored to the current radio
conditions experienced by the individual mobile station.
[0017] HSDPA and HSUPA furthermore use the following techniques in
order to reduce the delay (latency) associated with the data
communication: [0018] Short transmission time intervals.
Specifically, data transport blocks are sent to the transmitter at
frequent time intervals thereby allowing for transmissions and
retransmissions to be transmitted with a minimum of delay. [0019]
Scheduling and retransmission functionality located at the base
station. This may reduce the delay associated with scheduling and
retransmissions as control and data need not be communicated
between the RNC and the base station. [0020] Increased capacity.
The reduced resource usage and associated increased capacity in
itself reduces the delay incurred by buffering of data as a higher
data capacity may provide a higher throughput and thus reduced
queue sizes.
[0021] However, despite these techniques, the performance is not
optimal. Specifically, in conventional systems an operating point
is selected to provide an acceptable capacity and latency
performance. Although such an operating point may provide
acceptable performance in general, it is not optimal for many
situations and may in particular result in a relatively high
latency. For example, although an increased capacity may reduce
queuing delays, it may also increase other delays such as the delay
associated with retransmissions. Therefore, in order to achieve a
sufficiently high capacity, the retransmission delays may
frequently result in a latency which is higher than desired.
[0022] For example, in order to achieve a sufficiently high
capacity, it is important to transmit data packets at a
sufficiently low transmit power. The associated queuing delay is
thus reduced. However, this results in an increased block error
rate and thus an increased number of retransmissions being required
for successful communication. As the delay before a retransmission
occurs is substantial, this may substantially increase the
resulting average delay of transmitting data packets.
[0023] Hence, an improved system for communication would be
advantageous and in particular a system allowing for increased
flexibility, improved performance, reduced latency and/or increased
capacity would be advantageous.
SUMMARY OF THE INVENTION
[0024] Accordingly, the Invention seeks to preferably mitigate,
alleviate or eliminate one or more of the above mentioned
disadvantages singly or in any combination.
[0025] According to a first aspect of the invention, there is
provided an apparatus for a cellular communication system; the
apparatus comprising: a scheduler for scheduling data for
transmission over an air interface of the cellular communication
system using a retransmission scheme; load means for determining a
load characteristic associated with the scheduler; means for
setting a target parameter for the retransmission scheme in
response to the load characteristic; and means for setting a
transmission parameter for a transmission in response to the target
parameter.
[0026] The invention may provide for an improved performance of a
cellular communication system. In particular, the invention may
allow an improved performance wherein an operating point of a
retransmission scheme may be adapted to the current characteristics
of the cellular communication system. The invention may allow a
trade off between resource usage and delay to be dynamically
adjusted and/or may allow an improved trade off between queuing
delays and retransmission delays. For example, at low loads, the
transmission parameter may be set to result in a high resource
usage per transmitted information bit but low probability of
retransmissions thereby reducing retransmission delays whereas at
high loads, the transmission parameter may be set to result in a
low resource usage per transmitted information bit but at a higher
probability of retransmissions thereby reducing the queuing delay
and increasing capacity. In some embodiments, the latency
associated with communication of data may be reduced or
minimised.
[0027] The target parameter may specifically control the operating
point of the retransmission scheme. The functionality of the
apparatus may be distributed between different units and may in
particular be distributed between a fixed network of the cellular
communication system (including the base stations) and user
equipment of the cellular communication system. Thus, in some
embodiments, the apparatus may be distributed across an air
interface of the cellular communication system. For example, the
transmission may be a transmission by a user equipment whereas the
scheduler, the load means and/or the means for setting the target
parameter may be implemented in the fixed network and in particular
in a base station.
[0028] The load characteristic may be indicative of a loading of
the scheduler, of a loading of a cell, of a loading of a plurality
of cells or e.g. a loading of the cellular communication system as
a whole.
[0029] According to an optional feature of the invention, the
transmission parameter comprises a message transmit power. This
provides a suitable parameter to control the retransmission scheme
and allows a retransmission latency to be effectively and
advantageously controlled.
[0030] According to an optional feature of the invention, the
transmission parameter comprises a transmit power reference
indication. This provides a suitable parameter to control the
retransmission scheme and allows a retransmission latency to be
effectively and advantageously controlled. In particular, the power
reference indication may allow control of the retransmission
performance while allowing e.g. a link adaptation to also be
performed.
[0031] The power reference may be used by a power control mechanism
and/or a link adaptation mechanism.
[0032] For example, for a HSDPA or HSUPA service of a UMTS cellular
communication system, a power reference may be provided which is
used as a basis for the base station and/or remote unit to
determine a transmit transport format.
[0033] According to an optional feature of the invention, the
target parameter comprises an error rate. In particular, the error
rate may a Block Error Rate (BLER). The BLER may be a Packet Error
Rate (PER).
[0034] An error rate provides a particularly suitable parameter for
controlling an operation of a retransmission scheme. In particular
an error rate, such as the BLER, is a particularly suitable
parameter for controlling a retransmission probability and thus a
retransmission latency.
[0035] According to an optional feature of the invention, the
transmission parameter comprises a modulation parameter. The
modulation parameter may for example be a modulation order or a
spreading factor. A modulation parameter is particularly
advantageous for controlling transmissions and in particular
retransmission probabilities and may specifically be compatible
with existing systems such as the HSDPA and HSUPA services of
UMTS.
[0036] According to an optional feature of the invention, the
transmission parameter comprises an error coding parameter. The
error coding parameter may for example be a Forward Error
Correction (FEC) scheme to be applied to transmitted data. For
example, the FEC rate may be set in response to the target
parameter. In some embodiments, the transmission parameter may be
an indirect error coding parameter such as an indication of a
resource allocation and an amount of information data to be
communicated within the resource allocation. An error coding
parameter is particularly advantageous for controlling
transmissions and in particular retransmission probabilities and
may specifically be compatible with existing systems such as the
HSDPA and HSUPA services of UMTS. The transmission parameter may
specifically comprise a combined error coding and modulation
parameter.
[0037] According to an optional feature of the invention, the
transmission parameter comprises a transmission parameter set
restriction. This may provide a particularly suitable transmission
parameter for controlling the transmission performance and may in
particular allow an interference level to be limited. The
transmission parameter set restriction may restrict the set of
transmission parameters from which parameters for the transmission
can be selected. For example, in a HSDPA or HSUPA service of a UMTS
cellular communication system, the transmission parameter set
restriction may be a restriction of the Transmit Format
Combinations (TFCs) that can be used.
[0038] According to an optional feature of the invention, the
transmission parameter is a transmission parameter of an initial
transmission of a message. This may provide particularly
advantageous performance and efficient control. For example, the
probability of receiving the initial transmission in a
retransmission scheme has a major effect on the average
retransmission delay for the retransmission operation.
[0039] According to an optional feature of the invention, the
probability of a retransmission request is dependent on the setting
of the transmission parameter. The probability of retransmissions
may depend on the setting of the transmission parameter. The
invention may advantageously provide a means for controlling
retransmission characteristics and in particular a retransmission
latency by controlling a transmission parameter in response to a
target parameter.
[0040] According to an optional feature of the invention, the load
means is operable to determine the load characteristic in response
to an amount of pending transmit data. This provides an
advantageous measure of the loading of the system. In many
embodiments, a determination of the load characteristic in response
to the amount of pending data is particularly simple yet provides
an accurate indication of the current load.
[0041] According to an optional feature of the invention, the
pending transmit data is associated with a single cell. This may
allow for a particularly simple implementation in some embodiments.
In particular, it may in many embodiments allow a determination of
the load characteristic based only on information available at a
base station. Hence, the load characteristic may in some such
embodiments be determined individually by the base station.
[0042] According to an optional feature of the invention, the
pending transmit data is associated with a radio controller common
for a plurality of cells. This may allow for a particularly simple
implementation in some embodiments. In particular, it may in many
embodiments allow a determination of the load characteristic based
only on information available at the radio controller. Hence, the
load characteristic may in some such embodiments be determined
individually by the radio controller. The feature may additionally
or alternatively improve performance by controlling the
retransmission scheme in response to the conditions in the
plurality of cells.
[0043] The radio controller may specifically be a Radio Network
Controller (RNC).
[0044] According to an optional feature of the invention, the
amount of pending data corresponds to a transmit data buffer
loading. This may provide for a particularly low complexity
determination of the load characteristic and/or may provide a
particularly suitable load characteristic. The transmit data buffer
may for example be a transmit buffer of a radio network controller,
a base station and/or a remote unit.
[0045] According to an optional feature of the invention, the load
means is operable to determine the load characteristic in response
to a number of attached remote units.
[0046] This may provide a particularly suitable load characteristic
which may provide a suitable measure of the loading of cell(s) of
the cellular communication system. Alternatively or additionally,
this may provide for a low complexity determination of the load
characteristics and may in particular utilise readily available
information. For example, the higher the number of attached remote
units, the higher the loading may be considered to be.
[0047] The number of attached remote units may be determined in
response to a selection and/or filtering criterion. For example,
the number may be a number of active remote units in a single cell,
a number of remote units requesting service etc.
[0048] According to an optional feature of the invention, the
transmission parameter is biased towards an increasing number of
retransmissions for an increasing load. This may improve
performance, may allow an improved and dynamic trade off between
capacity and latency and/or may allow an improved trade off between
queuing latency and retransmission latency.
[0049] For example, at low loading the transmission parameter may
be set to have a negligible probability of retransmissions (e.g.
the BLER may be set very low). Accordingly, a high amount of
resource may be used but the retransmission latency may be reduced
significantly. At higher loadings, the additional resource will
typically not be available and the setting would result in reduced
capacity and increased queuing delays. Accordingly, in the example,
the transmission parameter is set to a higher probability of
retransmissions (e.g. the BLER may be set to a high value). This
will result in a lower resource usage (in particular when an H-ARQ
scheme is used) and thus reduced queuing delays at the expense of
an increased retransmission delay.
[0050] According to an optional feature of the invention, the
apparatus further comprises means for measuring a performance
characteristic; and the means for setting the target parameter is
further operable to set the target parameter in response to the
performance characteristic.
[0051] According to an optional feature of the invention, the
transmission is a downlink transmission. The transmission may in
particular be a HSDPA transmission.
[0052] According to an optional feature of the invention, the
transmission is an uplink transmission. The transmission may in
particular be a HSUPA transmission.
[0053] According to an optional feature of the invention, the load
means is operable to determine the load characteristic in response
to a load indication received from a remote unit. This may be
particularly suitable for uplink communication and may facilitate
implementation of functionality in a fixed network of the
communication system while allowing the determination to be in
response to characteristics of the remote unit. The load indication
may in particular be an indication of a transmit buffer loading of
the remote unit.
[0054] According to an optional feature of the invention, the
apparatus further comprises means for transmitting an indication of
the target parameter from a base station to a remote station. This
may in many embodiments facilitate or enable implementation and/or
a suitable distribution of functionality. In particular, it may
allow different elements to be implemented where it is most
suitable. For example, it may allow the load means and the means
for setting the target parameter to be implemented in the fixed
network while allowing the means for setting the transmission
parameter to be implemented in a remote unit.
[0055] According to an optional feature of the invention, the
apparatus further comprises means for transmitting an indication of
the transmission parameter from a base station to a remote station.
This may in many embodiments facilitate or enable implementation
and/or a suitable distribution of functionality. In particular, it
may allow different elements to be implemented where it is most
suitable. For example, it may allow the load means, the means for
setting the target parameter and the means for setting the
transmission parameter to be implemented in the fixed network while
controlling an uplink transmission.
[0056] In some embodiments, the indication of the target parameter
and/or the transmission parameter may be transmitted to the remote
station from a plurality of base stations. In some such
embodiments, the remote unit may ignore the indication unless the
same indication is received from a plurality of the base stations
(and in particular from all the base stations).
[0057] According to an optional feature of the invention, the means
for setting the transmission parameter is comprised in a base
station and the means for setting the target parameter is comprised
in a radio network controller which is operable to communicate an
indication of the target parameter to the base station. This may
provide a particularly advantageous implementation in some
embodiments. In particular, it may allow or facilitate the target
parameter being determined in a radio network controller based on
information, such as scheduling information, available at the radio
network controller while allowing efficient operation of the
retransmission scheme in the base station.
[0058] According to an optional feature of the invention, the
apparatus further comprises means for communicating the target
parameter to a scheduling function associated with a different cell
than a cell associated with the means for determining the target
parameter. This may facilitate operation in many embodiments and
may additionally or alternatively allow control of the
retransmission scheme in response to target parameters for other
cells.
[0059] According to an optional feature of the invention, the
apparatus further comprises means for communicating the load
characteristic to a scheduling function associated with a different
cell than the means for determining the load characteristic
parameter. This may facilitate operation in many embodiments and
may additionally or alternatively allow control of the
retransmission scheme in response to load characteristics for other
cells.
[0060] According to an optional feature of the invention, the
retransmission scheme is a Hybrid Automatic Repeat reQuest (H-ARQ)
scheme. The invention may provide improved performance for an H-ARQ
retransmission scheme. In particular, the soft combining of
transmissions may be used to increase capacity and reduce queuing
delays for high loadings whereas a more robust communication may be
used at lower loadings to reduce retransmission delays.
[0061] According to an optional feature of the invention, the
cellular communication system complies with the Technical
Specifications of the 3.sup.rd Generation Partnership Project
(3GPP). The transmission may in particular be a HSDPA or a HSUPA
data transmission.
[0062] According to a second aspect of the invention, there is
provided a base station for a cellular communication system; the
apparatus comprising: a scheduler for scheduling data for
transmission over an air interface of the cellular communication
system using a retransmission scheme; load means for determining a
load characteristic associated with the scheduler; means for
setting a target parameter for the retransmission scheme in
response to the load characteristic; and means for setting a
transmission parameter for a transmission in response to the target
parameter.
[0063] The optional features mentioned in relation to the apparatus
are equally applicable to the base station.
[0064] According to a third aspect of the invention, there is
provided a method of operation for a cellular communication system
including at least a scheduler for scheduling data for transmission
over an air interface of the cellular communication system using a
retransmission scheme, the method comprising: determining a load
characteristic associated with the scheduler; setting a target
parameter for the retransmission scheme in response to the load
characteristic; and setting a transmission parameter for a
transmission in response to the target parameter.
[0065] According to a fourth aspect of the invention, there is
provided an apparatus for a cellular communication system; the
apparatus comprising: a scheduler for scheduling data for
transmission over an air interface of the cellular communication
system using a retransmission scheme; load means for receiving a
load characteristic associated with the scheduler; means for
setting a target parameter for the retransmission scheme in
response to the load characteristic; and means for setting a
transmission parameter for a transmission in response to the target
parameter.
[0066] According to a fifth aspect of the invention, there is
provided an apparatus for a cellular communication system; the
apparatus comprising: a scheduler for scheduling data for
transmission over an air interface of the cellular communication
system using a retransmission scheme; means for receiving a target
parameter for the retransmission scheme, the target parameter being
dependent on a load characteristic associated with the scheduler;
and means for setting a transmission parameter for a transmission
in response to the target parameter.
[0067] According to a sixth aspect of the invention, there is
provided an apparatus for a cellular communication system including
a scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme; the apparatus comprising: load means for
receiving a load characteristic associated with the scheduler;
means for setting a target parameter for the retransmission scheme
in response to the load characteristic; and means for setting a
transmission parameter for a transmission in response to the target
parameter.
[0068] According to a seventh aspect of the invention, there is
provided an apparatus for a cellular communication system including
a scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme; the apparatus comprising: means for
receiving a target parameter for the retransmission scheme, the
target parameter being dependent on a load characteristic
associated with the scheduler; and means for setting a transmission
parameter for a transmission in response to the target
parameter.
[0069] According to an eight aspect of the invention, there is
provided an apparatus for a cellular communication system including
a scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme; the apparatus comprising: load means for
receiving a load characteristic associated with the scheduler;
means for setting a target parameter for the retransmission scheme
in response to the load characteristic; the target parameter being
indicative of a setting of a transmission parameter for a
transmission.
[0070] According to a ninth aspect of the invention, there is
provided a method for a cellular communication system including a
scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme; the method comprising: receiving a load
characteristic associated with the scheduler; setting a target
parameter for the retransmission scheme in response to the load
characteristic; and setting a transmission parameter for a
transmission in response to the target parameter.
[0071] According to a tenth aspect of the invention, there is
provided a method for a cellular communication system including a
scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme; the method comprising: receiving a target
parameter for the retransmission scheme, the target parameter being
dependent on a load characteristic associated with the scheduler;
and setting a transmission parameter for a transmission in response
to the target parameter.
[0072] According to an eleventh aspect of the invention, there is
provided a method for a cellular communication system including a
scheduler for scheduling data for transmission over an air
interface of the cellular communication system using a
retransmission scheme; the method comprising: determining a load
characteristic associated with the scheduler; and setting a target
parameter for the retransmission scheme in response to the load
characteristic; the target parameter being indicative of a setting
of a transmission parameter for a transmission.
[0073] The optional features mentioned in relation to the apparatus
are equally applicable to the method of operation.
[0074] These and other aspects, features and advantages of the
invention will be apparent from and elucidated with reference to
the embodiments described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0075] An embodiment of the invention will be described, by way of
example only, with reference to the drawings, in which
[0076] FIG. 1 is an illustration of part of a UMTS cellular
communication system in accordance with embodiments of the
invention;
[0077] FIG. 2 illustrates an example of packet data transmission in
a Hybrid ARQ retransmission scheme;
[0078] FIG. 3 illustrates an apparatus for transmission in
accordance with embodiments of the invention;
[0079] FIG. 4 illustrates an example set of transmit format
combinations for a high speed packet service;
[0080] FIG. 5 illustrates an example of uplink power control for a
high speed packet service;
[0081] FIG. 6 illustrates an example of link adaptation;
[0082] FIG. 7 illustrates an example of an operation of uplink
power control in accordance with embodiments of the invention;
[0083] FIG. 8 illustrates an example of transmit and receive powers
in an exemplary embodiment of the invention; and
[0084] FIG. 9 illustrates an example of a block error rate as a
function of signal to interference ratio.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0085] The following description focuses on an embodiment of the
invention applicable to a UMTS cellular communication system and in
particular to an application for a HSDPA or HSUPA service. However,
it will be appreciated that the invention is not limited to this
application but may be applied to many other cellular communication
systems and/or services.
[0086] FIG. 1 is an illustration of part of a UMTS cellular
communication system 100 in accordance with embodiments of the
invention.
[0087] FIG. 1 illustrates a Radio Network Controller (RNC) 101
which is connected to two base stations 103, 105 which are known as
Node Bs in a UMTS cellular communication system. The interface
between the RNC 101 and the base stations 103, 105 are known as the
Iub interface 107. The base stations 103, 105 support a number of
remote units 109, 111, 113 (or user equipment). For clarity and
brevity, FIG. 1 illustrates two remote units 109, 111 being
supported by the first base station 103 and one remote unit 113
being supported by the second base station 105. It will be
appreciated that base stations typically support a large number of
remote units in practical systems. The air interface between the
base stations 103, 105 and the remote units 109, 111, 113 is in
UMTS known as the Uu interface 115.
[0088] A remote unit may typically be a subscriber unit, a wireless
user equipment, a mobile station, a communication terminal, a
personal digital assistant, a laptop computer, an embedded
communication processor or any communication element capable of
communicating over the air interface.
[0089] In the example, the RNC 101 allocates a set of resources to
each base station 103, 105 which can be used by the base station
103, 105 exclusively for high speed packet services. The base
stations 103, 105 schedule transmissions to the remote units 109,
111, 113, operate a retransmission scheme and perform link
adaptation.
[0090] In particular, the UMTS communication system 100 uses a
retransmission scheme known as a Hybrid-ARQ (H-ARQ) retransmission
scheme which is an enhancement of the classical ARQ protocol.
[0091] A classical ARQ protocol operates by the receiver requesting
retransmission of erroneously received packets. In such a classical
retransmission scheme, the initially transmitted packet is deleted
from the receiving entity's memory before the retransmitted packet
is received. Thus the probability of correct reception of the
retransmitted packet is substantially the same as the probability
of correct reception of the initially transmitted packet when radio
conditions are the same for the initial transmission and the
retransmission.
[0092] In contrast, when a packet is erroneously received in an
H-ARQ protocol using soft combining, the receiving entity stores
soft information on the reliability of the bits of the erroneously
received packet and combines the soft information or the
reliability of the retransmitted packet with that stored
information. When the receiver decodes the combined soft
information, the probability of correct reception of this combined
reception is substantially greater than the probability of correct
reception of the retransmitted packet on its own.
[0093] It is well known that HARQ increases system throughput when
the system is operated at a high block error rate target for
individual transmissions. This is achieved by the system minimising
the total required transmission energy (including initial- and
retransmissions) for every data packet. By increasing the
transmitted energy little-by-little, and by checking the integrity
of the received data at each energy increment, the system ensures
that only just-enough energy is transmitted to ensure correct
reception of the packet.
[0094] FIG. 2 illustrates an example of packet data transmission in
a Hybrid ARQ retransmission scheme.
[0095] In FIG. 2, an example is illustrated where the classical ARQ
protocol requires three units of transmit energy to achieve correct
reception at the receiver with a probability of P.sub.Classic. This
reception occurs at time T.sub.1 following a transmission with the
required transmit energy.
[0096] In the example of the hybrid ARQ protocol, one unit of
transmit energy is used for each transmission from the base
station. After the first transmission at T.sub.0, the probability
of correct reception is P.sub.harq1, which is generally much less
than P.sub.Classic. The H-ARQ protocol retransmits packets that are
incorrectly received and the receiver collects all the energy from
transmissions and retransmissions until the packet can be decoded
(or the base station instructs the receiver to abandon attempts to
decode the packet). Thus at time T.sub.2, the receiver has received
2 units of transmit energy and is able to decode the packet
correctly with a probability of P.sub.harq2. After a third
transmission, three units of transmit energy have arrived at the
receiver (an identical amount of transmit energy to that which
arrived at the receiver in the case of the classical ARQ protocol).
The probability of correct reception for a decoding operation
performed at time T.sub.3 (both previous attempts have failed) is
now equal to (or greater than) the probability of correct reception
in the classical ARQ protocol case.
[0097] In the example, the probability of correct reception after
one transmission (at time T.sub.1) in the case of the H-ARQ
protocol is thus P.sub.harq1. Thus for a fraction P.sub.harq1 of
packets, only one unit of transmit energy is required for correct
reception of the packet (though for a fraction (1-P.sub.harq1), two
or more units of transmit energy are required for correct reception
of the packet). Hence for a fraction (1-P.sub.harq1)P.sub.harq2 of
packets, two units of transmit energy are required and for a
fraction (1-P.sub.harq1)(1-P.sub.harq2)P.sub.classic, three units
of transmit energy are required.
[0098] For example, when P.sub.harq1=0.5, P.sub.harq2=0.9 and
P.sub.classic=0.999, the expected transmit energy required for
correct reception of a packet is (ignoring the contributions from
three or more retransmissions):
(0.5P.sub.tx)+(0.5.times.0.9.times.2P.sub.tx)+(0.5.times.0.1.times.0.999.-
times.3P.sub.tx)=1.5498P.sub.tx where P.sub.tx is the unit of
transmit energy. The transmit energy required in the classical ARQ
protocol is 3P.sub.tx (ignoring the contribution from
retransmissions).
[0099] The amount of transmit energy required for the hybrid ARQ
protocol is thus almost half the energy required for the classical
ARQ protocol in this example (1.5498/3=0.5166).
[0100] In a power limited system, if only half the transmit power
is required to service each user, approximately twice as many users
may be supported. Use of an H-ARQ protocol can thus increase
capacity and throughput substantially.
[0101] However, the increased capacity is achieved at the expense
of increased retransmission latency. For the specific example of
FIG. 2, and assuming that T.sub.2=T.sub.1+t and T.sub.3=T.sub.1+2t
where t is the cycle time for retransmissions, the expected time to
correct reception of the packet is:
0.5(T.sub.1-T.sub.0)+0.5.times.0.9.times.(T.sub.1+t-T.sub.0)+0.5.times.0.-
1.times.0.999.times.(T.sub.1+2t-T.sub.0)=T.sub.1-T.sub.0+0.55t
[0102] The increased latency for H-ARQ operation can thus be very
significant (the cycle time, t, of the H-ARQ protocol is generally
significantly larger than the time T.sub.1-T.sub.0 since the time t
includes the time required for the receiver to signal that it
received a packet incorrectly back to the base station). The
latency of the H-ARQ protocol can thus typically be twice that of
the classical ARQ protocol, i.e.
(T.sub.1-T.sub.0+0.55t/T.sub.1-T.sub.0).apprxeq.2 for many
practical values of t and T.sub.1-T.sub.0.
[0103] Thus, the use of a hybrid ARQ retransmission scheme may
result in considerably larger system capacities at the expense of
increased retransmission latency. This increased system capacity
and increased latency is in particular achieved for increasing
block error rates of each transmission. The increase in block error
rate may itself be achieved via a decrease in the power transmitted
per block or an increase in the number of bits transmitted per
block at a given power level.
[0104] In communication systems such as UMTS, a significant delay
or latency may also result from a capacity limitation. For example,
for downlink communication, when a packet is to be sent from a
source the data packet is typically buffered or queued at several
places in the system. E.g. a packet to be transmitted is sent from
the source to an RNC. The RNC contains RLC (Radio Link Control)
buffers (the RLC functionality implements a high layer ARQ
protocol). Data is buffered in these RLC buffers until it can be
transmitted to the scheduling function in the base station. The
scheduling function is for HSDPA known as a MAC-hs (Media Access
Control-hs scheduler.
[0105] The base station contains multiple MAC-hs scheduling buffers
(there is at least one MAC-hs scheduling buffer for each remote
unit that is to be serviced by the base station). The base station
is connected to the remote units via the air interface. In HSDPA,
the base station controls a shared resource: the high speed
downlink shared channel (HS-DSCH). It is evident that for a given
amount of data, that data will spend a longer mean time in the
MAC-hs scheduling buffers when the aggregate transmission rate over
the air interface is lower. Similarly, data will be buffered for a
longer time in the RNC RLC buffers when the rate across the Iub
interface (linking the RNC and base station) is lower. Since the
rate across the Iub interface is at least partly dictated by the
air interface bottleneck, the length of time that data is buffered
in the RLC buffers is a function of the aggregate air interface
throughput.
[0106] It is thus evident that an increased system capacity can
reduce queuing latency. Thus, in communication services such as
HSDPA or HSUPA, a trade off may exist between queuing latency and
capacity on one side and retransmission latency on the other.
[0107] In conventional systems, the HARQ retransmission scheme is
designed to operate at a high capacity by operating at a high
target block error rate. In such systems, the retransmission cycle
time (i.e. the time between an initial transmission and a
retransmission or between retransmissions) is reduced as much as
possible in order to reduce the retransmission latency. However,
this remains undesirably high.
[0108] In accordance with some embodiments of the current
invention, a target parameter for the retransmission scheme, such
as a BLock Error Rate (BLER), is dynamically varied in response to
a load characteristic.
[0109] FIG. 3 illustrates an apparatus 300 for transmission in
accordance with embodiments of the invention.
[0110] The apparatus comprises a transmitter 301 for transmitting
data packets over the air interface. The data packets may be
initial data packets or may be retransmissions.
[0111] The apparatus furthermore comprises a transmit buffer 303
which receives and buffers data which is to be transmitted over the
air interface by the transmitter 301.
[0112] The transmit buffer 303 is coupled to a scheduler 305 which
schedules data for transmission over the air interface. The
scheduled data packets are fed to a transmission controller 307
which sets parameters of the transmission by the transmitter 301
such that a desired operation is obtained. In particular, the
transmission controller 307 comprises functionality for operating a
retransmission scheme, such as a H-ARQ scheme. The transmission
controller 307 may further comprise functionality for link
adaptation. Thus, in some embodiments, the transmission controller
307 may select a given set of transmission format parameters such
as a transmit power, modulation scheme, forward error correcting
scheme etc.
[0113] The apparatus 300 also comprises a load processor 309 which
is arranged to determine a load characteristic associated with the
scheduler 305. The load characteristic indicates a loading of the
cellular communication system which may affect or be affected by
the scheduling performed by the scheduler. The load characteristic
may for example be an indication of a current loading of the cell
for which the scheduler 305 schedules data.
[0114] The load characteristic may typically be an absolute or
relative load characteristic. For example, the load characteristic
may indicate a loading relative to a cell capacity. The cell
capacity may in some such embodiments be a predetermined static
parameter or may be dynamically determined based on
measurements.
[0115] The load processor 309 is coupled to a target controller 311
which is operable to set at least one target parameter for the
retransmission scheme in response to the load characteristic. The
target controller 311 may for example set a desired BLER for the
retransmission scheme. The target controller 311 is coupled to the
transmission controller 307 which is fed the target parameter. The
transmission controller 307 accordingly determines at least one
transmission parameter in response to the target parameter.
[0116] Hence, in embodiments according to FIG. 3, the initial
transmission and retransmission operation is controlled in response
to a load characteristic for the system. In other words, the
scheduling function performed by the scheduler 305 and transmission
controller 307 is modified as a function of the load
characteristic.
[0117] It will be appreciated that although the scheduler 305 and
the transmission controller 307 for clarity and brevity are shown
as separate entities performing separate functions, this is merely
an example. In many embodiments, the combined scheduling function
performed by the scheduler 305 and the transmission controller 307
is performed by the same processing element. Also in many
embodiments, the scheduling function comprises a combined
scheduling of new data, scheduling of retransmission data and
setting of appropriate transmission parameters.
[0118] In the example of FIG. 3, the load processor 309 and the
target controller 311 generate a target parameter for the
retransmission scheme which is used by the scheduling function to
set at least one transmission parameter as a function of the target
parameter. In particular, the load processor 309 and the
transmission controller 307 may change the operating point of the
retransmission scheme in response to the load characteristic.
[0119] For example, at high loads a BLER target parameter may be
set to a high value. When operating with a high target block error
rate, the transmitter will transmit less transmit power per data
packet, but will generate more re-transmissions. This will result
in increased retransmission latency but will also result in an
increased resource efficiency and a higher throughput and will
therefore reduce the queuing latency. As this is more significant
than the retransmission latency at loads approximating saturation,
a reduced overall delay is achieved at the higher load.
[0120] At low loads, a BLER target parameter may be set to a
relatively low value. When operating at a low target BLER, the
transmitter will transmit more power and generate fewer
retransmissions. This will result in significantly reduced
retransmission latency as retransmissions will only rarely be
required for successful communication of a data packet. However,
typically an excessive transmit power will be used thereby
resulting in an excessive resource usage. However for a low
loading, there is typically large amounts of unused resource which
would otherwise not be used. Furthermore, as the queuing delay is
typically negligible at low loads, the overall latency may be
reduced substantially.
[0121] Hence, in embodiments, the target parameter may dynamically
be varied to provide an improved operating point for the
retransmission scheme thereby providing improved performance. In
particular, the latency may be significantly reduced at low loads
without affecting the capacity of the system or the latency at high
loads. The overall latency may be reduced and in particular, the
trade off between the queuing and retransmission latency may be
dynamically adjusted to suit the current conditions. Similarly, the
trade-off between capacity and/or resource usage and latency may be
dynamically adjusted to or optimised for the current conditions.
Thus, the throughput perceived by the individual users may be
increased across a full range of system loadings.
[0122] For example, in many embodiments, queuing latency is
dominant over retransmission latency at high loads and in
particular when the load is approaching the available capacity.
However, at low loads, retransmission latency is typically dominant
over the queuing latency. According to some embodiments of FIG. 3,
the target parameter may thus be adjusted to provide optimal
operation for the queuing latency or for the retransmission
latency. In comparison to a conventional static operation, a
reduced latency may thus be achieved for both higher and lower
loads than the load associated with a static operating point. In
particular, at low loads, the latency may be reduced to the minimum
possible latency.
[0123] FIG. 3 illustrates functional blocks of the apparatus of
some embodiments of the invention. The individual functional blocks
may for example be implemented in a suitable processor such as a
microprocessor, a microcontroller or a digital signal processor.
The functions of the illustrated blocks may for example be
implemented as firmware or software routines running on suitable
processors or processing platforms. However, some or all of the
functional blocks may be implemented fully or partially in
hardware. For example, the functional blocks may be fully or
partially implemented as analog or digital circuitry or logic.
[0124] The functional blocks may furthermore be implemented
separately or may be combined in any suitable way. For example, the
same processor or processing platform may perform the functionality
of more than one of the functional blocks. In particular, a
firmware or software program of one processor may implement the
functionality of two-or more of the illustrated functional blocks.
For example, the load processor 309 and target controller 311 may
be implemented as different firmware routines running in a single
processor. The functionality of different functional modules may
for example be implemented as different sections of a single
firmware or software program, as different routines (e.g.
subroutines) of a firmware or software program or as different
firmware or software programs.
[0125] The functionality of the different functional modules may be
performed sequentially or may be performed fully or partially in
parallel. For example, a single firmware program may perform the
operation of the load processor 309 followed by the operation of
the target controller 311 followed by the operation of the
scheduler 305 followed by the operation of the transmission
controller 307. As another example, one processor may perform the
operation of the load processor 309 and the transmission controller
307 whereas another processor (or another processing element) may
perform the functionality of the scheduler and the transmission
controller 307. Parallel operation may include a partial or full
time overlap between the performed functions.
[0126] The buffer 303 is typically implemented by memory including
dynamic or static or semi-permanent semiconductor memory. For
example, the buffer may be implemented by integrated or separate
Random Access Memory (RAM) or (Programmable) Read Only Memory
(ROM). The buffer 303 may also fully or partially be implemented by
other memory types including magnetic and/or optical memories
including hard disk or optical disc based memories.
[0127] The functional elements may be implemented in the same
physical or logical element and may for example be implemented in
the same network element such as in an RNC, a base station or a
remote unit. In other embodiments, the functionality may be
distributed between different functional or logical units. For
example, the load processor 309 and target controller 311 may be
implemented in an RNC while the scheduler 305 and/or the
transmission controller 307 and the transmitter 301 may be
implemented in a base station. As another example, some functional
elements may be located in fixed network of the cellular
communication system whereas other elements may be located in
remote units. For example, the load processor 309 and the target
controller 311 may be implemented in a base station or an RNC, the
scheduler 303 may be implemented in the base station while the
transmit controller 307 and transmitter 301 are located in a remote
unit.
[0128] The functionality of individual functional units may also be
distributed between different logical or physical elements. For
example, the scheduler 305 may include functionality performed by a
scheduling in an RNC and a scheduling performed in a base
station.
[0129] It will be appreciated that any suitable target parameter
including combined target parameters comprising targets for a
plurality of individual parameters may be used.
[0130] In many embodiments, an error rate target parameter may
provide efficient control of the retransmission operation and allow
an efficient communication. In particular, the target parameter may
comprise or consist in a Block Error Rate (BLER). The BLER may
specifically be a Packet Error Rate (PER) of the transmission
blocks (data packets) of the retransmission scheme.
[0131] The operating point of the retransmission scheme may
effectively be controlled by the BLER target parameter and in
particular the BLER will provide an effective parameter for
controlling the number of retransmissions and the efficiency of the
resource usage. Thus, by setting a BLER target, the trade off
between capacity and retransmission latency and/or between queuing
latency and retransmission latency may be effectively
controlled.
[0132] For example, at low loading, there are typically excess
amounts of available resource. Accordingly, the target BLER may be
set to a low value of e.g. 10.sup.-4 resulting in a high resource
usage but in very few retransmissions and thus very low
retransmission latency. As the loading is low, the queuing latency
is insignificant and the total latency is thus substantially
minimised. As the loading increases, the excessive resource usage
results in a limited throughput which increases the queuing
latency. When the loading approaches the capacity of the system (or
cell), the queuing latency may increase substantially and the
capacity may furthermore be limited. Accordingly, as the load
increases, the BLER target may also be increased. For example, at
high loads, the BLER may be set to 0.2 resulting in the resource
usage being reduced for each data packet (facilitated by soft
combining of retransmissions). The reduced resource usage may
substantially increase the throughput of the system thereby
substantially reducing the queuing delays. Hence, although the
retransmission delay is increased, the capacity may be increased
and the queuing latency and the total delay may be reduced.
[0133] The target parameter may for example be determined in an
open loop arrangement. For example, a look up table associating
values of the load characteristic with values of the target
parameter may be stored in the target controller 311. This look up
table may for example be determined by simulations.
[0134] In some embodiments, the target parameter may be determined
in a closed loop arrangement. For example, the closed loop protocol
may adjust the relationship between the load characteristic and the
target parameter in order to maximise system capacity, minimise
latency and/or to maximise a function of system capacity and
latency. An example function would be
f.sub.sched=C.sup..lamda./L.sup..eta.where .lamda. and .eta. are
adjustment parameters and C is the capacity and L is the
latency.
[0135] It will be appreciated that one or more suitable
transmissions parameters may be determined by the transmission
controller 307 in response to the target parameter.
[0136] In many embodiments, the transmission parameter may
advantageously be the error coding and modulation applied to a
message which is to be transmitted. The message may for example be
an initial transmission of a data packet or may be a retransmission
of a data packet. The probability of a retransmission is dependent
on the selected error coding and modulation and by selecting a
suitable error coding and modulation, the transmission controller
307 may adjust the transmission to match the target parameter. For
example, for a BLER target parameter, the transmission controller
307 may determine the error coding and modulation which for the
current propagation conditions and interference level at the
receiver (e.g. measured by the receiver and transmitted back to the
transmitter) will result in the desired BLER.
[0137] The transmission parameter may alternatively or additionally
comprise other transmission parameters. For example, the BLER of
the transmitted data packet typically depends on transmit power
used and the spreading ratio of the spreading code used. Hence,
these parameters may in some embodiments be determined to provide
the desired retransmission operating point.
[0138] In some embodiments, the transmit parameter may be a
combined transmission parameter comprising a set of transmission
format parameters including for example a transmit power, a
modulation scheme, an error coding scheme and/or a spreading
factor.
[0139] It will be appreciated that the load characteristic may be
determined in accordance with any suitable algorithm and in
response to any suitable parameter(s) and/or measurements.
[0140] In some embodiments, the load processor 309 determines the
load characteristic in response to an amount of pending transmit
data. Typically, for increasing amounts of transmit data that is to
be transmitted by e.g. the transmitter 301, the queuing delays and
the requirement for an efficient resource usage increases. Hence,
the load processor 309 may determine an indication of how much data
is to be transmitted, and the target controller 311 may set the
target parameter in response thereto. For example, a BLER target
may be increased for increasing amounts of pending transmit
data.
[0141] The pending transmit data may be determined in response to a
buffer level. For example, the load processor 309 may be coupled to
the buffer 303 and may determine the load characteristic as the
current buffer loading. The target controller 311 may then set the
target parameter in response to the buffer loading.
[0142] In some embodiments, the pending data may be determined in
response to a buffer level in an RNC and/or the buffer loading in a
base station and/or the buffer loading in a remote unit.
Especially, the buffer loading of RNC scheduling buffers may e.g.
be used if the load processor 309 and the target controller 311 are
located in an RNC. The buffer loading of base station scheduling
buffers may e.g. be used if the load processor 309 and the target
controller 311 are located in a base station. The buffer loading of
a remote unit may for example be used for uplink
communications.
[0143] The pending transmit data may in some embodiments be
associated with a plurality of cells. In other embodiments, the
transmit data may be associated with a single cell. This may allow
an adaptation to local conditions and/or may facilitate
implementation, for example when the load processor 309 and target
controller 311 are implemented in a base station.
[0144] In some embodiments, the load processor 309 may additionally
or alternatively determine the load characteristic in response to a
number of attached remote units. The number of attached remote
units may for example be a number of attached remote units which
meet a specific criterion such as the number of remote units which
actively request service or the number of attached remote units
which are in a single cell.
[0145] The information of the number of attached remote stations in
a given cell is available at the RNC and the base station serving
that cell. Accordingly, this approach may be practical in
embodiments wherein the load processor 309 is located in an RNC or
base station. However, in other examples, the information of the
number of attached users may be communicated through the
interconnecting network. A remote unit is typically attached if it
is registered in a non-idle state with a base station.
[0146] In the following, specific embodiments of the invention
applicable to an HSUPA uplink service of a UMTS cellular
communication system will be described. The embodiments are
applicable to the example of FIG. 3 and will be described with
reference to this.
[0147] In the embodiments, the target controller 311 modifies a
BLER target for the hybrid ARQ protocol dynamically based on
measurements of the offered load. In the embodiments, the offered
load measurement is measured from buffer volume measurements sent
to the base station by the remote unit. Between receptions of
buffer volume measurements, the offered load can be interpolated by
the base station.
[0148] In HSUPA, link adaptation is performed by the remote unit
dynamically varying the transmit (or transport) format which is
used for the transmissions. The remote unit is signalled a set of
Transport Format Combinations (TFCs) which it can use for
transmissions. These transport format combinations form a Transport
Format Combination Set (TFCS). Each TFC in the TFCS carries a
different amount of data and requires a different amount of
transmit power. In current UMTS networks, the channel coding rate
applied to each TFC within the TFCS is essentially the same, and
hence the different TFCs within the TFCS indicate pairs of number
of data bits and amounts of physical resource, but the ratio of
number of bits to physical resource is essentially constant within
a TFCS. In general, the concept of TFCS can be extended to allow
TFCs with different coding rates within the TFCS. The power
required for each TFC within the TFCS (to obtain the same
probability of correct reception as all other TFCs in the TFCS) is
expressed as an offset from a base power level.
[0149] An example set of TFCs within a TFCS is shown in FIG. 4. The
figure shows an example set of TFCs for a UMTS system. The coding
rate of each TFC within the TFCS is the same--as more transport
blocks are transmitted per TFC, the code resource of the TFC
increases. The diagram shows a reference power level termed
P.sub.base and a set of .beta. values that are power offsets from
P.sub.base. Addition of the .beta. values to P.sub.base indicates
the power required to support a given TFC. The .beta. values may
either be signalled to the remote unit by the network or calculated
by the remote unit.
[0150] A mobile radio channel experiences channel impairments
including fading. A consequence of fading is that if the remote
unit transmits at a constant power level, the power level received
by the base station will fluctuate as a consequence of the fading.
This fluctuation in received power level can cause several
problems: [0151] when a reduced amount of power is received from
the remote unit, the signal to interference ratio of the reception
may be insufficient to correctly decode the transmission. [0152]
when an increased amount of power is received from the remote unit,
this increased power may interfere with transmissions from other
remote units.
[0153] Power control techniques are employed in UMTS to counteract
these effects. In general, a failure to respond to these power
fluctuations (no power control) will increase the required
transmitted energy per bit for a given block error probability.
[0154] Firstly, the network is able to signal to the remote unit a
maximum TFC that the remote unit can use. Restricting the maximum
TFC restricts the maximum power that the remote unit can transmit.
This allows the network to ensure that an excessive amount of power
is not received from a remote unit. Thus this method allows
interference at the base station to be controlled.
[0155] Secondly, the network is able to control the P.sub.base
level at the remote unit. In FDD, the base power level P.sub.base
is changed by power control commands (TPC bits) transmitted by the
network: when the network signals a TPC=down command on a downlink
channel, the remote unit reduces the value of P.sub.base used in
the uplink (similarly, when "up" is signalled, the remote unit
increases the value of P.sub.base). In TDD, the base power level
P.sub.base is calculated by the remote unit based on a target SIR
level, a base station interference level signalled by the network
and a path loss measured by the remote unit (note that channel
reciprocity can be assumed in TDD).
[0156] Operation of uplink power control is shown in FIG. 5. FIG. 5
shows the channel fading profile and the corresponding value of
P.sub.base at the remote unit. FIG. 5 also shows the powers
required to transmit each of the TFCs shown in FIG. 4. FIG. 5 shows
a maximum transmit power limit, P.sub.uem, for the remote unit
which is equal to the minimum of the maximum allowed transmission
power (as set by the network) and the maximum power capability of
the remote unit. The network may choose to reduce the maximum
allowed transmit power in the presence of excessive uplink
interference. The maximum transmit power capability of the remote
unit is typically based on factors such as the battery power of the
remote unit and the design of the transmit radio (the remote unit
is typically designed to operate its radio frequency components in
their linear ranges; any attempt to transmit too much power is
likely to cause the RF components to operate in a non-linear range
which can either damage the RF circuitry or cause the RF to radiate
unwanted radio frequency emissions).
[0157] In typical system operation, the remote unit alters the TFC
that it transmits according to the fading channel profile (and the
hence to the changing value of P.sub.base). In FIG. 5, the remote
unit changes the TFC that it transmits between time T.sub.1 and
T.sub.2 in order not to exceed its maximum remote unit transmit
power (when the channel fading profile worsens, the remote unit
reduces the TFC that it transmits and maintains maximum transmit
power. It then relies on the channel coding and spreading to
overcome the increasing fade depth as it maintains its power
constant). Between time T.sub.2 and T.sub.3, the remote unit is
able to use its maximum signalled TFC (TFC3: in this example, the
network has disallowed use of TFC4 due to interference reasons).
During this time, the remote unit reduces its transmit power in
order to conserve battery power and reduce interference at the
network.
[0158] The resultant power transmitted by the remote unit and
received at the base station is shown in FIG. 6. During the period
T.sub.1 to T.sub.2, the remote unit is transmitting at, or close
to, maximum remote unit transmit power. During period T2 to T3, the
remote unit transmits in its maximum allowed TFC (TFC3) and reduces
the power applied to its transmission as the fade depth decreases,
thus maintaining a constant received power at the base station.
FIG. 6 shows that use of the restriction on maximum TFC ensures
that the received power from the remote unit is below the critical
value, P.sub.interf, at which damaging interference will be caused
by the remote unit received power (this damaging power can either
create excessive interference in the cell in which the remote unit
is transmitting or in both the cell in which the remote unit is
transmitting and in neighbouring cells or only in some neighbouring
cells).
[0159] In the example of FIG. 6, the remote unit changes from using
TFC3 to using TFC2 at time T.sub.1 due to increasing fade depth in
the channel. Initially, the remote unit has more power than is
required to support this TFC (it has more than enough power to
support TFC2, but not enough power to support TFC3); the remote
unit thus reduces its transmit power at time T.sub.1. Following T1,
the fade depth of the channel increases, thus the remote unit
increases its transmit power towards its maximum transmit power.
When the remote unit reaches its maximum transmit power, it changes
TFC again to TFC1.
[0160] Note that the power received by the base station from the
remote unit decreases as the remote unit switches to lower TFCs.
The lower TFCs have greater inherent reliability due to increased
processing gain, thus the base station is able to decode these
lower TFCs despite their lower received power.
[0161] In an exemplary uplink embodiment, the scheduler 305 and
transmission controller 307 operate on the basis of buffer volume
measurements and other parameters such as estimates of the uplink
channel quality. The buffer volume measurements indicate the amount
of data that is pending for transmission in remote unit data
buffers. These buffer volume measurements are signalled from the
remote unit to the network. The network may estimate remote unit
buffer volume in between measurement reports from the remote
unit.
[0162] In the embodiment, the target controller 311 modifies a BLER
target as the buffer volume measurements indicate greater offered
load by remote units attached to the network. As network load
increases, the target controller 311 indicates an increasing a BLER
target.
[0163] In the example, a TFC restriction and a power-modifying
command (such as SIR target or TPC) is sent from the transmission
controller 307 in the base station to the remote unit. Hence, in
the specific example, the target controller 311 and transmission
controller 307 are implemented in the fixed network (including the
base stations) whereas the transmitter 301 and buffer 303 are
implemented in the remote unit. In the example, the link selection
of the TFC used is performed by the transmitter 301 in the remote
unit taking into account the restriction transmitted by the base
station. The functionality of the transmission controller 307 may
in some examples be distributed between the fixed network and the
remote unit.
[0164] In the case of TDD (where in this section, it is assumed
that the uplink power can in general be controlled either by
signalling SIR target changes or by the use of TPC commands), when
the block error rate target is increased, the scheduler reduces the
amount of resource allocated to each remote unit since it knows
that remote units will use higher coding (and potentially
modulation) formats when there is a higher block error rate target.
In order to increase capacity, when the network reduces the amount
of resource allocated to each remote unit, it also decreases the
transmission power required for any given TFC for those remote
units (by lowering P.sub.base, either by modifying an SIR target or
by use of appropriate TPC commands). By reducing the amount of
resource allocated to each remote unit and reducing the power for
each remote unit, more remote units can be accommodated within the
total resource available to the base station (where in this case,
"resource" implies code/timeslot utilisation and allowed generated
interference) and these remote units will be able to support the
same data rate as previously (but at a higher latency). This
increases system capacity at the cost of retransmission
latency.
[0165] The transmission controller 307 may alternatively or
additionally signal a higher TFC restriction and a lower value of
P.sub.base to the remote unit while applying the same other
resources to the transmission (where a lower value of P.sub.base
can be signalled either by modifying an SIR target or by the use of
appropriate TPC commands). This has the effect of increasing the
coding rate (and potentially the modulation format) that the remote
unit can use at the same transmit power (thus creating the same
uplink interference in the network). Use of a higher TFC
restriction and a lower value of P.sub.base increases the data rate
that remote units can support while maintaining the same
interference level at the base station at the cost of higher
retransmission latency: the same number of remote units can be
supported as previously but each is transmitting at a higher data
rate. Use of a higher TFC restriction only increases the data rate
that remote units can support but increases the interference level
at the base station: the same number of remote units can be
supported as previously but each is transmitting at a higher data
rate.
[0166] In the case of FDD, when the block error rate target is
increased, the transmission controller 307 may increase the maximum
TFC restriction to the remote unit (allowing the remote unit to use
higher coding rates and potentially modulations) and lower the
value of P.sub.base that the remote unit operates at (this may be
achieved by sending extra "down" TPC commands to the remote unit).
By sending these extra "down" TPC commands and increasing the
maximum TFC for the remote unit, the base station causes the remote
unit to transmit at a higher coding rate (and potentially
modulation) while creating the same amount of interference as when
the lower block error rate target is employed.
[0167] The transmission controller 307 may alternatively increase
the TFC restriction while maintaining the P.sub.base restriction.
This alternative has the effect of allowing the remote unit to
transmit at a higher power. These higher power transmissions create
more interference at the base stations and have the effect of
increasing the BLER in the network. In this case, the same number
of remote units can be supported as previously, but each is
transmitting at a higher data rate at the cost of a higher
retransmission latency.
[0168] As an alternative, in FDD, the base station can maintain the
maximum TFC restriction when it increases the BLER target and
lowers the value of P.sub.base that the remote unit operates at.
This alternative has the effect of reducing the interference that
any one remote unit creates and allows the base station to support
more remote units for the same total amount of base station
interference.
[0169] Thus, the transmission controller 307 may determine a
transmit power reference indication, such as P.sub.base, and use
this to control the retransmission operation. Additionally or
alternatively, the transmission controller 307 may determine a
transmission parameter set restriction, such as a restriction of
the TFCs which may be used by the remote unit, and use this to
control the retransmission operation.
[0170] FIG. 7 illustrates an example of an operation of uplink
power control in accordance with the example. In particular, FIG. 7
illustrates operation of uplink power control in both the lightly
loaded case (low BLER, high P.sub.base) and in the heavily loaded
case (high BLER, low P.sub.base). The value of P.sub.base is
altered in the network by the methods described previously (by
lowering the SIR target or by sending extra "down" TPC commands to
the remote unit). In FIG. 7, the maximum TFC for the lightly loaded
case is TFC3 and for the heavily loaded case it is TFC4.
[0171] FIG. 7 illustrates that when the value of P.sub.base is
reduced by the network (in the heavily loaded case), the TFCs that
are used by the remote unit increase. Note also that the remote
unit is able to transmit using TFC4 at a greater fade depth than it
could transmit using TFC3 in the lightly loaded case: this
justifies the network decision to increase the TFC restriction from
TFC3 to TFC4 (in addition to lowering P.sub.base) when the network
loading increases.
[0172] FIG. 8 illustrates the transmit and receive powers at the
remote unit according to the example. In particular, FIG. 8 shows
the lightly loaded case when the target controller 311 sets a low
BLER target (and high P.sub.base) and the heavily loaded case when
the target controller 311 sets a high BLER target (and low
P.sub.base). It can be seen that in the heavily loaded case, a
higher TFC is typically used than in the lightly loaded case under
the same channel conditions. Use of this higher TFC does not
increase interference in the system since this higher TFC (e.g.
TFC4) is transmitted at the same power in the heavily loaded system
(with low P.sub.base) as a lower TFC (e.g. TFC3) in the lightly
loaded system. However, an increased amount of data may be
transmitted at the higher TFC.
[0173] FIG. 8 illustrates that the received power at any given TFC
is lower in the heavily loaded system. The BLER of the heavily
loaded system is thus greater than that of the lightly loaded
system.
[0174] It should be noted that FIG. 8 illustrates the method of
increasing system capacity by raising the maximum TFC restriction.
If the maximum TFC restriction were not increased (for example, if
a maximum TFC of TFC3 were used), then the power received from the
remote unit by the base station would be less in the heavily loaded
system than in the lightly loaded system. The lower received power
from the remote unit would create less interference at the base
station. Since each remote unit would create less interference in
the heavily loaded system (operating at a lower P.sub.base), the
base station could admit more users onto the system before a
critical level of interference is reached at the base station.
[0175] In some uplink embodiments, the target controller 311 may be
implemented in the fixed network and the transmission controller
307 may be fully or partially implemented in the remote unit. In
such embodiments, the base station may explicitly signal to the
remote unit that a change in H-ARQ operating point is to be applied
(the remote unit may thus be in control of the setting of the H-ARQ
operating point).
[0176] Specifically, the network may signal to the remote unit that
a new BLER target is to be applied. This signalling may for example
in a UMTS system be by RRC signalling from the RNC in the network
or by lower layer (MAC or PHY layer) fast signalling from the
network (typically the base station).
[0177] In the following, specific embodiments of the invention
applicable to an HSDPA downlink service of a UMTS cellular
communication system will be described. The embodiments are
applicable to the example of FIG. 3 and will be described with
reference to this.
[0178] In the example, the apparatus seeks to provide maximum
capacity and minimum latency for a HSDPA service. In the example,
the target controller 311 varies the BLER target as the offered
load in the system changes. The offered load may be measured either
as the number of admitted users or the amount of data that is
stored in the network's buffers (base station and/or RNC
buffers).
[0179] Specifically the BLER target may be varied as a function of
the total amount of data buffered in the base station. The BLER
target can be modified dynamically as the measure of buffered data
changes (for example, during busy hours, there may be more buffered
data and the target controller 311 will increase the BLER target in
order to increase throughput). The relationship between the amount
of buffered data and BLER target is carefully calculated (either
off-line and a-priori, or dynamically within the base station) to
substantially minimise the latency across users and hence maximise
the throughput perceived by them.
[0180] A typical block error rate vs C/I (signal to interference
ratio) curve is shown in FIG. 9.
[0181] As a specific example, when a cell is lightly loaded, the
target controller 311 will (for example) signal a block error rate
target of 0.001 to be used by the transmission controller 307. If
the channel C/I is 7 dB, rate 1/3 QPSK is used. Although this is a
low coding rate and modulation format, this is acceptable since the
scheduler 305 and transmission controller 307 can allocate a lot of
physical resource (codes and timeslots) to the remote unit in order
to satisfy the data rate requirements of the remote unit as there
is little load on the network (if the physical resources were not
allocated to this remote unit, they would not be allocated at all
since there are no other remote units requesting service in the
lightly loaded example). Use of a block error rate target of 0.001
minimises latency since most packets are received correctly by the
remote unit at the first attempt.
[0182] However, in the example, when the cell is heavily loaded,
the target controller 311 increases the BLER target to 0.5. When
the channel C/I is 7 dB, rate 1/2 16 QAM is used. Note that for any
given amount of physical resource, three times the number of bits
can be transmitted with rate 1/2 16 QAM than can be transmitted
with rate 1/3 QPSK. Use of this high coding rate and modulation
minimises the amount of physical resource required to transmit the
packet. This increases system capacity and increases latency due to
re-transmissions but can reduce queuing latency.
[0183] It will be appreciated that it is also possible to operate
the system such that when the block error rate target is increased
from 0.001 (channel C/I=7 dB, rate 1/3 QPSK) to 0.5, the power
applied to the code may be decreased to -2 dB while maintaining the
rate 1/3 QPSK coding and modulation scheme.
[0184] It will be appreciated that the specific operating points
described are examples only and that it is of course possible to
use different operating points to those described here. Indeed, it
is possible for the target controller 311 to dynamically change the
BLER target over a continuous range. Additionally, other system
parameters or metrics than a BLER target may be used to control the
transmission error probabilities.
[0185] In some embodiments, the target controller 311 may further
be operable to set the target parameter in response to a
performance characteristic. For example, a performance of a given
service may be measured and used to adjust the target parameter. As
an example, the RNC can measure the elapsed time between data
packets being received in the RNC transmit buffers and
acknowledgements of correct receipt of these packets being received
from the remote units (this performance characteristic is a latency
measurement). The RNC then feeds these latency measurements into a
control loop that adjusts the target parameter with a view to
minimising the measured latency. The RNC may then signal this
target parameter to the base station.
[0186] In some embodiments, target parameters or load
characteristics associated with one cell, base station or scheduler
may be communicated to scheduling functions with other cells, base
stations or schedulers.
[0187] For example, the apparatus 300 may be implemented in a first
base station and may control the retransmission operation within a
first cell served by the base station. However, a similar
functionality may be implemented in a second base station for
controlling the retransmission operation within a second cell
served by the second base station. In such a case, the load
characteristics and/or the target parameter may be exchanged
between the base stations. This may allow the setting of the target
parameter in one cell to be dependent on the characteristics in
another cell.
[0188] Especially, if the first and second cells are neighbouring
cells, the radio environment of one cell will affect the other
cell. For example, excessive resource usage in one cell may
introduce excessive interference to the other cell thereby reducing
the capacity and possibly increasing queuing lengths. Hence, by
increasing a BLER target in one cell in response to a load
characteristic in another cell the latency in the other cell may be
reduced or the capacity increased.
[0189] In some such embodiments, signalling may be established
between base stations to indicate that a given absolute or relative
target parameter is to be used. An algorithm may be implemented to
allow base stations to come to an agreement on the target parameter
to be used. The communication may be directly from base station to
base station or may be via the RNC.
[0190] In some embodiments, an RNC may signal to a plurality of
base stations to operate with a given absolute or relative target
parameter. For example, in response to an increasing load
indication determined at an RNC for a plurality of cells, the RNC
may signal a higher BLER target to all the cells served by the RNC
thereby improving the resource efficiency and interference in all
cells.
[0191] In some embodiments, a remote unit may receive signals from
or transmit signals to a plurality of base stations. For example, a
remote unit may be in a soft handover. In situations where the
target parameter is transmitted from the base stations to the
remote unit, the remote unit may be operable only to change the
target parameter used if a sufficient number of receptions from the
plurality of base stations are in agreement. For example, it may
only change the target parameter if all or a majority of received
target parameters are identical. A similar approach may be applied
to transmissions from the remote unit.
[0192] It will be appreciated that the above description for
clarity has described embodiments of the invention with reference
to different functional units and processors. However, it will be
apparent that any suitable distribution of functionality between
different functional units or processors may be used without
detracting from the invention. Hence, references to specific
functional units are only to be seen as references to suitable
means for providing the described functionality rather than
indicative of a strict logical or physical structure or
organization.
[0193] The invention can be implemented in any suitable form
including hardware, software, firmware or any combination of these.
However, preferably, the invention is implemented at least partly
as computer software running on one or more data processors and/or
digital signal processors. The elements and components of an
embodiment of the invention may be physically, functionally and
logically implemented in any suitable way. Indeed the functionality
may be implemented in a single unit, in a plurality of units or as
part of other functional units. As such, the invention may be
implemented in a single unit or may be physically and functionally
distributed between different units and processors.
[0194] Although the present invention has been described in
connection with some embodiments, it is not intended to be limited
to the specific form set forth herein. Rather, the scope of the
present invention is limited only by the accompanying claims.
Additionally, although a feature may appear to be described in
connection with a particular embodiment, one skilled in the art
would recognize that various features of the described embodiments
may be combined in accordance with the invention. In the claims,
the term comprising does not exclude the presence of other elements
or steps. Furthermore, although individually listed, a plurality of
means, elements or method steps may be implemented by e.g. a single
unit or processor. Additionally, although individual features may
be included in different claims, these may possibly be
advantageously combined, and the inclusion in different claims does
not imply that a combination of features is not feasible and/or
advantageous. Also the inclusion of a feature in one category of
claims does not imply a limitation to this category but rather
indicates that the feature is equally applicable to other claim
categories as appropriate. In addition, singular references do not
exclude a plurality. Thus references to "a", "an", "first",
"second" etc do not preclude a plurality.
* * * * *